Contrast amplification in global texture orientation discrimination

We show that adding a low-contrast texture stimulus that is far below its own detection threshold to an ambiguously oriented high-contrast texture can produce an easily perceived global orientation. When such a low-contrast (e. g., 0.1%) test texture and a high-contrast (e. g., 2%) amplifier texture are interleaved, the effective strength for global orientation detection closely approximates the product of the two contrasts. Therefore, adding two ambiguous textures, an amplifier texture at 5 x its threshold contrast for global orientation discrimination and a test texture at 1/5x its threshold contrast, produces threshold global orientation discrimination, that is, 5 x amplification of the below-threshold test texture. The observed 5 x amplification factors are larger than facilitation effects reported in other pattern tasks. Amplification is 11 x when orientation discrimination thresholds are compared to absolute detection thresholds. For second-order textures, maximum contrast amplification is about 2.5 x. A contrast gain control model is presented that accounts for 90% of the variance in observed d' for texture patterns of differing geometries, exposure durations, and component contrasts. In the model, very low-contrast orientations are represented by power functions of their contrasts, with an exponent greater than two. As the contrast of an amplifier texture increases beyond about 4%, feed-forward gain control exerted by the amplifier ultimately nullifies the amplification effect and produces masking.